Process of reducing density of fast surface states in MOS devices

ABSTRACT

Fast surface states in MOS devices, such as SCCDs, are reduced by depositing a relatively thin amorphous layer containing silicon and hydrogen onto the SiO 2  surface of such devices and annealing the resultant device in a non-oxidizing atmosphere for brief periods of time at a temperature in excess of the deposition temperature for the amorphous layer but below about 500° C. so that free valences at the Si-SiO 2  interface region are saturated with hydrogen. Surface state densities of about 4×10 8  cm -2  eV -1  and SCCDs having ε=1.10 -5  can be achieved via this process. The process is useful in producing SCCDs with low surface state densities and other MOS devices having low surface generated dark currents.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to MOS devices and somewhat more particularly to aprocess of reducing density of fast surface states in MOS devices, suchas surface-charge-coupled devices (SCCDs).

2. Prior Art

In surface-charge-coupled devices, for example as described by C. H.Sequin et al, Charge Transfer Devices, Academic Press Inc., New York,pages 11 and 12, (1975), low surface state densities at Si-SiO₂interface regions are of primary importance in regards to transmissionproperties (and dark currents) associated with such devices.

Normally, the production of MOS devices comprises a careful gateoxidation followed by hydrogen annealing in a gas atmosphere. B. E.Deal, "The Current Understanding Of Charges In The Thermally OxidizedSilicon Structure", J. Electrochem. Soc., Vol. 121, No. 6, pages 198Cthrough 205C (1974) suggests that hydrogen saturates so-called"dangling" bonds at Si-SiO₂ interface regions in semiconductorstructures. During such saturation, the dissociation of the H₂ moleculesinto atomic hydrogen is of primary importance. This saturation occursduring annealing in a hydrogen atmosphere at a temperature in the rangefrom about 400° C. to 500° C. over a time period of about 30 minutes.Surface state densities which can be achieved by means of this processare about (5-10)×10⁹ cm⁻² eV⁻¹. In the case of SCCDs this means atransfer loss, ε, of about (1-2)×10⁴, which is inadequate for manyapplications.

SUMMARY OF THE INVENTION

The invention provides improved process for reducing surface statedensities in MOS devices and in optimizing the transfer properties ofSCCDs devices.

In accordance with the principles of the invention, after completion ofa MOS device which includes a Si-SiO₂ interface region with the SiO₂defining a free surface and having electrodes therein, a relatively thinamorphous dielectric layer comprised of silicon and hydrogen isdeposited onto the free SiO₂ surface and the resultant structure isannealed in a non-oxidizing atmosphere at a temperature above thedeposition temperature for the amorphous layer but below 500° C. Thisannealing process breaks up the Si-H bonds in the amorphous layer andreleases the atomic hydrogen for saturating free valences at the Si-SiO₂interface region.

In accordance with the preferred embodiments of the invention, thedeposition of the amorphous layer occurs via a CVD reaction (chemicalvapour deposition) in an electric low-pressure glow discharge systemwhich preferably includes a plate reactor, with the gas pressure withinsuch system being adjustable over the range of about 50 to 300 mTorr. Inpreferred embodiments, the deposition temperature utilized in depositingthe amorphous layer ranges from about 100° C. to 450° C.

In preferred embodiments of the invention, the amorphous layer isdeposited from a reaction gas comprised of silanium (SiH₄) which, incertain embodiments may be admixed with a nitrogen-containing gas,preferably selected from the group consisting of nitrogen, ammonia andmixtures thereof. In this manner, the amorphous layer, in addition tocontaining silicon and hydrogen, also contains nitrogen so that duringthe subsequent annealing process, the amorphous layer is prevented frombeing transformed into a polycrystalline state.

In preferred embodiments of the invention, the amorphous layer thicknessranges from between about 1000 to 10,000 angstroms. Further, inpreferred embodiments of the invention, the atomic ratio of silicon tohydrogen in the amorphous layer varies between about 2 to 10 and inembodiments where nitrogen is incorporated within such amorphous layer,the maximum atomic ratio of nitrogen to silicon is about 1.3.

In preferred embodiments of the invention, the non-oxidizing atmosphereutilized during annealing is comprised of a gas selected from the groupconsisting of hydrogen, inert gases and mixtures thereof and theannealing occurs over a time period of at least about 5 minutes.

By optimizing the principles of the invention, surface state densitiesof about (6±2)×10⁸ cm⁻² eV⁻¹ are readily achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic two-dimension sketch of the prior art process forsaturating free valences in a Si-SiO₂ interface region with hydrogen;

FIG. 2 is a somewhat similar schematic representation of a processoccurring in accordance with the principles of the invention; and

FIG. 3 is a curve diagram of typical surface state densities, N_(SS), inthree similar devices treated in different manners.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a process of reducing surface state densities inMOS devices and in optimizing the transfer properties of SCCDs.

In accordance with the principles of the invention, after the completionof a select device structure, a relatively thin amorphous dielectriclayer containing silicon and hydrogen is deposited on the devicesurface, which has been provided with electrodes, and the resultantstructure is then annealed in a non-oxidizing atmosphere for relativelybrief periods of time at a temperature above the amorphous layerdeposition temperature and below 500° C.

The amorphous layer deposition process is carried out during theproduction of desired MOS devices in addition to the normaldouble-polysilicon process. Prior to the annealing step, contact windowscan be etched into the amorphous silicon-hydrogen layer by conventionalphotoresist techniques so that, for example, gold wire contacts can beprovided to the device prior to the annealing step.

The reduction of surface state densities is governed decisively by theannealing process which is carried out in a non-oxidizing atmosphereafter the deposition of the amorphous layer. The non-oxidizingatmosphere is preferably composed of a gas selected from the groupconsisting of hydrogen, an inert gas, mixtures thereof and mostpreferably nitrogen. The amorphous layer is deposited at a selectdeposition temperature from a suitable reaction gas, preferablycomprised of silanium (SiH₄), which in certain embodiments can bediluted with a nitrogen-containing gas, such as N₂, NH₃ and mixturesthereof. The inclusion of nitrogen in the amorphous layer prevents theamorphous layer from being transformed into a polycrystalline stateduring the subsequent annealing process.

The amorphous layer is deposited in a thickness ranging from out 1000 to10,000 A. The atomic ratio of silicon to hydrogen in the amorphous layerranges from about 2 to 10 and in embodiments where nitrogen is presentin such layer, the atomic ratio of nitrogen to silicon increases from 0to 1.3.

By following the principles of the invention, the Si-H bonds in theamorphous layer are broken-up during the annealing procedures so thatatomic hydrogen is released and directly diffuses to the SiO₂ -Siinterface region where it can complex or saturate the free siliconvalences. With an optimization of the principles of the invention, MOSdevices having surface state densities of about (6±2)×10⁸ cm⁻² eV⁻¹ areattained.

Referring now to the drawings, FIG. 1 illustrates a SiO₂ -Si interfaceregion 10, with an electrode zone 12 provided on the outer or free SiO₂surface undergoing hydrogen saturation from a suitable gaseous hydrogensource, H₂, whereby atomic hydrogen passes through the electrode zone12, the SiO₂ zone and saturates the "dangling" silicon bonds, asindicated by the broken lines 11. As can be seen, not all free valencesof silicon are saturated via this process and some fast surface states,N_(SS), remain.

FIG. 2 illustrates a somewhat similar SiO₂ -Si interface zone 10 of aMOS device having an electrode zone 12 on the free surface of the SiO₂region, which is coated with an amorphous layer 13. In the embodimentillustrated, amorphous layer 13 is comprised of an admixture of silicon,nitrogen and hydrogen and is represented by the empirical formula Si_(x)N_(y) H_(z) wherein x, y and z are numerals. As can be seen, with theprocess of the invention substantially all free valences of silicon aresaturated so that a substantial reduction in the density of the fastsurface states is attained.

In the two representations shown at FIGS. 1 and 2, the followingreactions occur:

At FIG. 1, with H₂ gas:

    .tbd.Si--+H.sub.2 →.tbd.Si--H+.sup.+ H              (1)

    .tbd.Si--+.sup.+ H→.tbd.Si--H                       (2)

With reaction (1) above occurring very slowly.

At FIG. 2, with a Si_(x) H_(y) N_(z) amorphous layer:

    Si.sub.x H.sub.y N.sub.z →si.sub.x H.sub.y-1 N.sub.z +.sup.+ H (3)

    .tbd.Si--+.sup.+ H→.tbd.Si--H                       (4)

The optimum values achievable with the practice of the principles of theinvention can be derived from FIG. 3, which is a curve diagram oftypical surface state densities, N_(SS), measured by the conductancemethod with MOS capacitors. Section (a) indicates that value of thesurface state density measured on a device after typical prior arthydrogen annealing, whereas section (b) indicates the surface statedensity value after applying an amorphous layer comprised of silicon andhydrogen on such device and annealing such device in accordance with theprinciples of the invention and section (c) illustrates the surfacestate density value obtained by following the principles of theinvention, with the annealing occurring in a nitrogen atmosphere.

As can be seen from the FIG. 3 diagram in an exemplary presentlypreferred embodiment, an MOS device having a surface state density,N_(SS) cm⁻² eV⁻¹ of 4×10⁸ cm⁻² eV⁻¹ was achieved.

The principles of the invention are applicable to all SCCDs and all MOSdevices with surface generated dark currents. The charge losses, ε of1×10⁻⁵ measured on SCCDs produced in accordance with the principles ofthe invention represent an important prerequisite for the use of suchdevices as highly integrated filters, memories, sensors, etc. Thetransfer properties thus attained are substantially optimal. As a resultof the low surface state densities, the proportion of dark current dueto surface generation is extremely small. In normal CCD operation of adevice produced in accordance with the principles of the invention, thedark current is a factor of 3 smaller than in previously availabledevices.

As is apparent from the foregoing specification, the present inventionis susceptible of being embodied with various alterations andmodifications which may differ particularly from those that have beendescribed in the preceding specification and description. For thisreason, it is to be fully understood that all of the foregoing isintended to be merely illustrative and is not to be construed orinterpreted as being restrictive or otherwise limiting of the presentinvention, excepting as it is set forth and defined in thehereto-appended claims.

We claim as our invention:
 1. A process for reducing the density of fastsurface states in MOS devices, such as surface-charge-coupled devices(SCCDs), comprising:providing a MOS device having a Si-SiO₂ interfaceregion with the SiO₂ defining a free surface of such device and havingelectrodes therein; depositing a relatively thin amorphous layercomprised of silicon, hydrogen and nitrogen from a reaction gascomprised of silanium and a nitrogen-containing gas at a givendeposition temperature onto said SiO₂ free surface of the device; andannealing the resultant structure in a non-oxidizing atmosphere for arelatively brief period of time at a temperature in excess of said givendeposition temperature and below about 500° C. whereby free valences atthe Si-SiO₂ interface region are saturated with hydrogen and the densityof fast surface states is reduced.
 2. A process as defined in claim 1wherein said nitrogen-containing gas is selected from the groupconsisting of N₂, NH₃ and mixtures thereof.
 3. A process as defined inclaim 1 wherein the maximum atomic ratio of nitrogen to silicon in saidamorphous layer is about 1.3.